The positive effect of plant diversity on soil carbon depends on climate

Little is currently known about how climate modulates the relationship between plant diversity and soil organic carbon and the mechanisms involved. Yet, this knowledge is of crucial importance in times of climate change and biodiversity loss. Here, we show that plant diversity is positively correlated with soil carbon content and soil carbon-to-nitrogen ratio across 84 grasslands on six continents that span wide climate gradients. The relationships between plant diversity and soil carbon as well as plant diversity and soil organic matter quality (carbon-to-nitrogen ratio) are particularly strong in warm and arid climates. While plant biomass is positively correlated with soil carbon, plant biomass is not significantly correlated with plant diversity. Our results indicate that plant diversity influences soil carbon storage not via the quantity of organic matter (plant biomass) inputs to soil, but through the quality of organic matter. The study implies that ecosystem management that restores plant diversity likely enhances soil carbon sequestration, particularly in warm and arid climates.


Fig. S1. Global map depicting mean annual precipitation and the 84 grassland sites (orange dots).
The map was created in R (version 4.2.1) using the packages ggmap (version 3.0.2) and OpenStreetMap (version 0.3.4).

Fig. S2. Coefficient of determination (R 2 ) of linear models of soil organic carbon (SOC) and
Shannon index as well as carbon-to-nitrogen (C:N) ratio and Shannon index for different subsets of sites selected according to climate variables.Points depict the R 2 of the linear model of SOC and Shannon index (a, c, e) as well as C:N ratio and Shannon index (b, d, f) for 26 subsets of sites as a function of the mean of the climate variable of each subset (mean annual temperature (MAT; a, b), mean annual precipitation (MAP; c, d), and aridity index (e, f)).The smallest subset in each panel (marked in blue) consists of eight sites with either the highest MAT, lowest MAP or lowest aridity index.The size of the subsets was increased in a stepwise manner by decreasing MAT or by increasing MAP or aridity index (adding one site per step).The largest subset (marked in beige) consists of 33 sites.Points marked by a cross (×) indicate statistically significant models (P < 0.05).The point of each panel that is marked by a black triangle corresponds to the linear model shown in Figure 2 or 3. Horizontal lines at the bottom of the panels show the MAT, MAP or aridity index ranges of each of the 26 subsets.The horizontal line and the point of each subset have the same color.Note that by definition, the aridity index increases with decreasing aridity.

Fig. S3 .
Fig. S3.Relationship between plant biomass and soil organic carbon content.The relationship is shown across all grassland sites (a) as well as across sites with mean annual temperature (MAT) > 15.58 °C (b).The linear models were plotted to the site-level data (and not to the plot data, which is shown to give insight into the variability).

Fig. S4 .
Fig. S4.Soil total organic carbon-to-nitrogen (C:N) ratio as a function of climate.Soil C: N ratio as a function of mean annual temperature (a), mean annual precipitation (b), and the aridity index (c) across 84 grasslands.The linear mode was plotted to the site-level data (and not to the plot data, which is shown to give insight into the variability).

Fig. S5 .
Fig. S5.Relationship between the Shannon index and soil nitrogen content.The relationship is shown across all 84 grassland sites (a) as well as across sites with mean annual temperature (MAT) > 15.58 °C (b), sites with mean annual precipitation (MAP) < 523 mm (c), and arid and semi-arid sites, i.e., sites with an aridity index (AI) < 0.50 (d).The linear models were plotted to the site-level data (and not to the plot data, which is shown to give insight into the variability).

Fig. S6 .
Fig. S6.Initial version of the new structural equation model.

Table S2 . Results of multiple regression analyses of soil organic carbon as a function of biomass and climate variables.
Significant interactions (P < 0.05) between biomass and climate variables are marked in bold font.SOC stands for soil organic carbon, MAT stands for mean annual temperature, MAP stands for mean annual precipitation, AI stands for aridity index.N refers to the number of observations (i.e.number of grassland sites).